The hydroxyl satellite-line `flip' as a tracer of expanding HII regions

Observations of the $^{2}\Pi_{3/2},~J = 3/2~$ground state transitions of the hydroxyl radical (OH) have emerged as an effective tracer of `CO-dark' molecular gas in diffuse regions of the Galactic interstellar medium (ISM). The transitions at 1612 and 1720 MHz in particular -- the `satellite lines' -- are sensitive tracers of local environmental conditions. As a powerful example of the utility of the satellite-line transitions we discuss a peculiar spectral feature known as the satellite-line `flip' wherein the satellite lines flip -- one from emission to absorption and the other the reverse -- across a closely blended double feature. We highlight 31 examples of the satellite-line flip from the literature and from previously unpublished data sets, 28 of which exhibit the same orientation with respect to velocity: the 1720 MHz line is inverted at more negative velocities, flipping to 1612 MHz inversion at more positive velocities. To explain these trends we propose a scenario where the alternating inversion seen in the flip arises from molecular gas both just inside and just outside a shock front surrounding an expanding HII region: the HII region provides the background continuum and its expansion accounts for the bias in velocity. We find that the flip can be reproduced across a wide range of number densities ($10^2-10^5\,{\rm cm}^{-3}$), a wide but high range of molecular gas temperatures ($50-150\,{\rm K}$), and requires moderate velocity dispersions ($<3\,$km s$^{-1}$) and column densities alternately above and below approximately $10^{15}\,{\rm cm}^{-2}$.